FIG. 1 of the accompanying drawings illustrates a known type of quadrature frequency converter. The frequency converter comprises two signal paths containing an in-phase (I) mixer 1 and a quadrature (Q) mixer 2. The mixer receives commutating signals, which are intended to be in phase-quadrature with respect to each other, from a local oscillator 3 via a quadrature splitter 4 and mixes the signal in each signal path with the respective commutating signal to generate in-phase and quadrature output signals IO and QO. The “quality” of the quadrature output signals is determined at least in part by amplitude imbalances between the signal paths and quadrature phase imbalances between the quadrature commutating signals.
Gain imbalances may be reduced on the order of 0.1 dB by good design and layout techniques. However, quadrature phase imbalances are more difficult to reduce to levels which provide acceptable quadrature frequency changing performance. For example, in arrangements where the frequency of the local oscillator 3 is required to vary by an octave or more, it is difficult to reduce phase imbalances to less than 3° across the frequency band of operation.
A known technique for reducing quadrature phase errors is to supply the quadrature commutating signals to a double-balanced or a double-double-balanced mixer. Such a mixer produces an error signal proportional to the phase error from 90° from the commutating signals and this error signal may be used to control a feedback loop including the quadrature splitter 4 so as to reduce phase imbalance. However, such an arrangement is relatively complex and inconvenient.
In FIG. 1, the phase error from 90° between the commutating signals is represented by a phase error or shift of φ° added to the in-phase or 0° commutating signal whereas the quadrature or 90° signal is uncontaminated. The commutating signals are illustrated as vectors in the graph of FIG. 2 in the accompanying drawings and, when applied to the mixers 1 and 2, result in the same phase imbalance or error in the converted output signals IO and QO. The phase error φ° has to be reduced to a value which is sufficiently low to provide acceptable operation of the quadrature frequency converter.